Re: [PATCH v6 2/2] Documentation: devicetree: Add boost-frequency binding to list boost mode frequency

On 05/30/2014 03:45 PM, Rob Herring wrote:

On Fri, May 30, 2014 at 3:33 PM, Nishanth Menon [off-list ref] wrote:

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On 05/30/2014 03:19 PM, Tomasz Figa wrote:

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On 30.05.2014 22:13, Nishanth Menon wrote:

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On 05/30/2014 03:02 PM, Tomasz Figa wrote:

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On 30.05.2014 21:50, Nishanth Menon wrote:

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On 05/30/2014 01:55 PM, Mark Rutland wrote:

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On Fri, May 30, 2014 at 07:05:43PM +0100, Thomas Abraham wrote:

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Hi Mark,

On Fri, May 30, 2014 at 6:38 PM, Mark Rutland [off-list ref] wrote:

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Hi,

Apologies for being somewhat late w.r.t. review on this.

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Why are these in both operating-points and boost-frequencies? It'll be
really easy to accidentally forget to mark something as a
boost-frequency this way. Why not have a boost-points instead?

Does boost-points mean a set of clock speeds which are not listed as
part of operating-points property? If yes, that also is a possible
implementation (it was implemented in one of the earlier version of
this series). Could you confirm that you want the boost mode speeds to
be exclusive of the speeds listed in operating-points?

If these boost mode operating points are not generally advisable for use
as the other operating-points are, then they should IMO been in an
entirely separate property exclusive of (but in the same format as) the
operating-points property, e.g.

operating points = <A B>, <C D>;
boost-points = <E F>;

you are asking boost frequencies to remain for ever tied down to OPP
style description.

What we are trying to describe? "What are my SoC's overclocked
frequencies"? That description can be used even in a system that does
not use OPP style table (say ACPI based OPP tables or whatever
acronymned table).

Tying it down to operating points just because we have it today as a
convenient description, is limiting.

Further, if we decide to educate boost-frequencies to also indicate
how long is it safe? That does indeed belong to boost-frequency
description and not OPP description. Or if we decide to change
operating-points description[1] in the future has an impact on
"boost-points" description, when it should not have.

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Otherwise, without boost-mode support we have to parse the boost mode
table to figure out which points to avoid. Or if someone typos a value

That is OS usage of h/w description - yeah - in anycase, if OS has no
ability to deal with boost-frequencies, it should skip it for sure.

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in either table we might go into a boost mode when we didn't want to!

There are other ways to screw up device with dts typo. you could give
a wrong voltage(extra 0?) and ensure you never use the chip ever
again.. typos are dt bugs, we can do the best to write robust code to
report them.

Typos are not the primary thing to worry about here. Adding boost
frequencies to the list of primary operating points is flawed, because
an OS that has no idea of boost mode will use them as normal operating
points and this is not desired.

That means we have an implementation bug in OS since it does not
consider the complete hardware description that device tree is
providing. An analogy will be a regulator compatible match being used
but regulator-min-microvolt and regulator-max-microvolt being ignored
by OS.

No. The operating points bindings were defined far before this
boost-frequency and so there is no requirement to support the latter.

So, we dont add any new bindings ever again? /me blinks. *IF* we add a
new property in the future, do we expect the new description to be
supported in older kernel(which could not have known about it)? How
far are we taking this ABI thing?
Documentation/devicetree/bindings/ABI.txt states:
3) Bindings can be augmented, but the driver shouldn't break when given
   the old binding. ie. add additional properties, but don't change the
   meaning of an existing property. For drivers, default to the original
   behaviour when a newly added property is missing.

we are not changing the meaning of existing property, we are
augumenting it.

You are changing the meaning of entries in that they can have
additional data which changes their properties.

If we accept the DT changes (as DT maintainers) and reject the kernel
changes (as kernel maintainers), you would be left with a broken
system.

That is true and I agree.

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In my opinion, *IF* we are concerned about polluting operating-points
description, why dont we enforce that the boost operating points
should NOT be defined in the current "operating-points" description -
and - just follow what Rob suggested and iMx already does - add such
operating points from platform code.

I believe I also said on this and other attempts to bandage up the
existing OPP binding, that we should not change/append it, but define
a new binding for OPPs that addresses this and other issues.
Otherwise, I'm going to just NAK every incomplete OPP binding bandaid.

Are we open to creating a completely mutually-exclusive binding and
maintain the legacy one as "legacy support without any modification"
while we debate the new binding? I dont deny that we have more bandage
discussions that we'd like on these bindings, So, would like views if
we want to define a better one from scratch rather than continue to
figure out how to live with existing one?

Here are the options I can think of.
1. New binding: Lets say each OPP as phandles? -> that sounds like the
closest I have heard as something that remotely makes sense to scale
properties?
2. Remove OPP bindings entirely - only way to add/modify OPP is from
platform code.

Anyone has better suggestions what we can do?

-- 
Regards,
Nishanth Menon